Long term evolution (LTE) wireless communication systems use orthogonal frequency division multiplexing (OFDM) in the downlink (base station to user equipment) and discrete Fourier transform (DFT)-spread OFDM in the uplink (user equipment to base station). The basic LTE downlink physical resource can thus be seen as a time-frequency grid 2 as illustrated in FIG. 1, where each resource element (RE) 6 corresponds to one OFDM subcarrier 4 during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames 10 of 10 ms, each radio frame consisting of ten equally-sized sub frames 8 of length Tsubframe=1 ms, as shown in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each sub frame the base station transmits control information indicating to which terminals data is transmitted and upon which resource blocks the data is transmitted in the current downlink sub frame. This control signaling is performed via physical channels such as the physical control channel (PDCCH), physical control format indicator channel (PCFICH), and physical hybrid automated request indicator channel (PHICH). The control signaling that indicates the target terminal(s) and the resource blocks is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each sub frame, across the system bandwidth. The number n=1, 2, 3 or 4 in which the control signaling is transmitted is known as the Control Format Indicator (CFI) transmitted in the PCFICH.
In LTE Release.11 of the 3rd generation partnership project (3GPP) standard, an enhanced PDCCH (EPDCCH) was introduced which does not map a control message across the system bandwidth but instead in a limited (N=2, 4, 8) number of physical resource block (PRB) pairs known as the EPDCCH set. The location of these PRB pairs in the set is configured to the UE using radio resource control (RRC) signaling. A Rel.11 user equipment (UE) can be configured with one or two EPDCCH sets for its control signaling. The EPDCCH spans the whole sub frame. However the starting OFDM symbol for EPDCCH has to be s=n+1 where n is the control format indicator (CFI), to avoid collision between the PDCCH/PCFICH/PHICH and the EPDCCH. The value of s can either be obtained from detecting CFI (i.e. n) or it can be configured to a fixed value by radio resource control (RRC) signaling. The EPDCCH cannot (in Rel.11) be used to transmit broadcast control messages. Only UE specific scheduling control messages can be transmitted. This is because only the UE specific search space (USS) is monitored on EPDCCH. Hence, the PDCCH is used for broadcast control messages (common search space) irrespectively of whether EPDCCH is configured to a Rel.11 UE or not. A pre-Rel.11 UE uses the PDCCH for both common and UE specific search space monitoring.
The downlink sub frame also contains a number of different reference symbols, which are known to the receiver and used for coherent demodulation of, for example, the control information. The PDCCH uses common reference symbols (CRS) for demodulation and these can, in Rel.11, have 1, 2 or 4 antenna ports. These common reference symbols span the whole system bandwidth, i.e. they are present in all PRB pairs, and are present in all sub frames even if no PDCCH is transmitted. The EPDCCH uses UE specific demodulation reference symbols (DMRS) for demodulation. These reference symbols are only present in the PRB pairs of the EPDCCH set if there is an EPDCCH transmission in the set in the specific sub frame. In case there is no EPDCCH transmission, the UE specific reference symbols (RS) are not transmitted.
For the physical downlink shared channel (PDSCH), there are ten transmission modes (TM) defined in LTE, as shown in Table 1 below. Each TM has two transmission schemes, where the second scheme is a “fallback scheme” that generally has only a single layer transmission. Some of the transmission modes use CRS-based transmit diversity for robustness, i.e., CRS is used as a demodulation reference. Scheme 2 is used when scheduling using downlink control information (DCI) format 1A which can be used in all TMs. Thus, scheme 2 is the transmission scheme that can be used during RRC reconfiguration of the TM for a UE. Some TMs use the CRS for PDSCH demodulation and others use the UE specific RS, also known as DMRS, as for example TM10. In a multi-broadcast single frequency network (MBSFN), sub frames only have CRS in the first OFDM symbol. In non-MBSFN sub frames, CRS is always used for scheme 2. However, TM9 and TM10 use UE specific RS whenever the sub frame is a MBSFN sub frame.
TABLE 1Available transmission modes for a REL.11 terminalRS/RERS used forRS used forused forTransmissiondemodulationdemodulationchannelmodeFeatureof scheme 1of scheme 2feedback1SingleCommon RSCommon RSCommonantennaRS2TransmitCommon RSCommon RSCommondiversityRS3Open loopCommon RSCommon RSCommonMIMO, maxRS4 layers4Closed loopCommon RSCommon RSCommonMIMO, maxRS4 layers5MU-MIMO,Common RSCommon RSCommonmax 1 layer,RSmax 2 UEs6Closed loopCommon RSCommon RSCommonMIMO, maxRS1 layer7TDDUE specificCommon RSCommonMIMO, maxRSRS1 layer8TDDUE specificCommon RSCommonMIMO, maxRSRS2 layers9Closed loopUE specificCommon RSCSI-RSMIMO, maxRSor UE8 layersspecific RS10Closed loopUE specificCommon RSCSI-RS +MIMO +RSor UEIMRCoMP, maxspecific RS8 layers
Also, RSs used for channel state information (CSI) feedback differ among the transmission modes. TM9 and TM10 utilize special channel state information RS (CSI-RS), which is more sparse than the CRS and cannot be used for demodulation. Instead, CSI-RS is used primarily for measuring channel state information. In TM10, the interference measurement resource (IMR) is introduced, which is a set of resource elements (REs) in which the UE is mandated to measure the interference used in computing the channel state information (CSI) report. For TM1-TM9, the interference measurement is unspecified, but most UE implementations use residual interference on the common reference signal (CRS) to create the interference covariance estimate.
The demodulation reference signal (DMRS) is only present in the PRB pairs containing PDSCH, while CRS is always present in all PRB pairs and all sub frames. A PRB pair 14 in a downlink system with CFI=3 OFDM symbols as PDCCH/PCFICH/PHICH control is illustrated in FIG. 3. FIG. 3 also shows the CRS 12.
UEs configured in TM10 and configured with EPDCCH use DMRS for demodulation of both PDSCH and EPDCCH but use the CRS for demodulating the PDCCH/PCFICH and PHICH. Furthermore, the use of PHICH is not strictly necessary, since HARQ indication for the uplink can be transmitted using the EPDCCH instead. Also, the PCFICH, which indicates the length of the region containing PDCCH, is unused in case the EPDCCH and PDSCH start position is configured by RRC. So the PHICH and PCFICH can actually be unused and are not needed to be received at all for terminals configured in TM10 (Rel.11 capability is required to support TM10). Thus, there is no reason in this case to transmit the PHICH and PCFICH.
Hence, only PDCCH is necessary to be transmitted to convey broadcast information. However, in case of carrier aggregation, only the primary cell (Pcell) is transmitting broadcast information, hence the PDCCH need not be present on the secondary cell (Scell). So in the case of an Scell, there is no PDCCH/PCFICH/PHICH needed. Thus, CRS are not used at all for demodulation purposes for the secondary cells.
There are also other means for providing broadcast information in the control plane (C-plane) to the UE under discussion by the 3 GPP standards body, such as dual connectivity. Hence, in some cases being considered by the 3GPP, there is no need for CRS for demodulation purposes. In these scenarios, where UEs get C-plane information from other sources, with only TM10 configured UEs on a carrier, the CRS is present in all sub frames and creates overhead since these REs used for CRS cannot be used for PDSCH or EPDCCH. Furthermore, the CRS creates unnecessary interference in a PBR pair even if the PRB pair is unused, i.e., not scheduled on the PDSCH.
A new carrier type, has been proposed that contains either no CRS at all or much less CRS either in frequency (by, for example, a reduction of the bandwidth the CRS covers to be smaller than the carrier bandwidth) or in time (by, for example, not transmitting any CRS in some pre-defined sub frames) or in both frequency and time, as compared to a legacy carrier. In the proposed new carrier type, CRS is transmitted in sub frame 0 and sub frame 5 for frequency and time synchronization tracking and radio resource management (RRM) measurements, but is not used for channel estimation or demodulation.
As shown in Table 1, a UE configured in TM9 or TM10 uses CSI-RS for channel state information feedback and UE specific RS for demodulation in case of scheme 1. For the “fallback” scheme 2, CRS is used for demodulation unless the sub frame is an MBSFN sub frame, in which case a single UE specific RS is used (port 7). Even though these two TMs are based on UE specific RS for demodulation, they utilize CRS for their channel analyzer estimates, at least for Doppler frequency and Doppler shift estimation.
A modified TM10, known as enhanced TM10 (eTM10) or TM11, has also been proposed. UEs configured in this new TM and receiving PDSCH can be dynamically configured (by an EPDCCH or PDCCH message) in a given sub frame to start the PDSCH at OFDM symbol 0 and/or to assume that no CRS are present in the sub frame. Hence, the overhead from CRS is removed. The CRS is overhead since it is not used by the UE either for feedback or demodulation. This leads to better spectral efficiency and higher UE throughput. Alternatively, when PDCCH and CRS are present, their presence can be indicated in downlink control signaling so that the PDSCH to RE mapping avoids the legacy control region and the CRS REs. Thus, the eTM10 mode, when combined with sub frame level discontinuous transmissions (DTX), allows UEs to receive DMRS based transmissions without any CRS in the sub frame, and for CRS transmissions to be turned off when there is no data sent in a sub frame. This enhanced TM10 mode and sub frame level DTX combination provides the possibility of fast on/off behavior of the base station transmissions and can be used to operate without CRS to achieve the same result as the proposed new carrier type when all UEs in the cell are capable of using these enhancements. Furthermore such an enhanced transmission mode can seamlessly accommodate one or more CRS ports in any sub frame.
Based on some current proposals, the CRS may be removed on all sub frames except sub frame 0 and 5 in a radio frame (containing 10 sub frames). The CRS is used in these sub frames to support time and frequency synchronization and for RRM measurements. In all other sub frames, the CRS are not transmitted. This enables downlink discontinuous transmission (DL DTX) in cases where there is no UE to be scheduled and there are no legacy UEs active in the cell (since the legacy UEs require CRS to be present in every sub frame).
Thus, in the enhanced TM10 proposal (eTM10), the CRS is absent from sub frames except for sub frames 0 and 5. This means that the channel analyzer that relies on CRS for at least Doppler related parameter estimation will have reduced performance. Also, the fallback transmission scheme, i.e. scheme 2, currently assumes CRS for demodulation when the sub frame is a non-MBSFN sub frame.
Another problem with TM10 is that it does not provide for robust transmission, such as transmit diversity and open loop MIMO using DMRS. For instance, the PDSCH is transmitted using a single antenna port. Hence, no transmit diversity is possible. If DL DTX is used on a carrier, then robust CRS based TMs like TM2 and TM3 may not be used. This is particularly a drawback in cases where feedback is unavailable due to large CSI-RS periodicity (which is likely to maximize the DTX gain), or when UE speed is high with respect to the CSI-RS periodicity.
A lack of CRS and/or PDCCH will make the proposed new type of carrier or transmission mode inaccessible by legacy release UEs when deployed, i.e., the proposed new type of carrier and transmission mode are not backward compatible. The proposed new type of carrier and transmission mode are referred to as carrier type or transmission mode type B. Carrier type or transmission mode A (legacy carrier or transmission mode) and carrier type or transmission mode B are illustrated in FIG. 4 and FIG. 5, respectively. FIG. 4 shows channels containing CRS 16, PCFICH/PDCCH/PHICH 18, BCH/PSS/SSS 19, and PDSCH 20 (which includes eDPCCH). FIG. 5 shows that the CRS 16 are restricted to a sub frame 22. A UE that supports carrier type or transmission mode B also supports carrier type or transmission mode A and these UEs are referred to as UE B. Legacy UEs are referred to as type A UEs.
A network typically configures the UE to assist the reception of various signals and/or channels based on different types of reference signals including, for example, CRS (represented as antenna ports 0-3), DMRS, i.e. antenna ports 7-14, CSI-RS, i.e. antenna ports 15-22. Reference symbols may possibly be exploited for estimation of propagation parameters and preferred transmission properties to be reported by the UEs to the network, e.g., for link adaptation and scheduling. In general, the channel from each antenna port to each UE receive port is substantially unique. However, some statistical properties and propagation parameters may be common to different antenna ports, depending on whether the different antenna ports originate from the same point or not. Such properties include, for example, the received power level for each port, the delay spread, the Doppler spread, the received timing (i.e., the timing of the first significant channel tap) and the frequency shift.
Channel estimation algorithms perform a three step operation. A first step includes the estimation of some of the statistical properties of the channel such as average delay spread and Doppler shift. This step can be done several sub frames before receiving data, and is commonly performed using a channel analyzer in the UE. The estimation can also be done based on a first type of reference signal that is different from a second type of reference signal, for which the estimate is needed, if the reference signals belonging to the two types are quasi co-located. This step can also be done by combining estimates from several sub frames, using a channel property tracking algorithm. A second step includes generating a channel estimation filter based on such estimated properties or parameters. A third step, which is carried out only when the UE is receiving data or control data, and needs a channel estimate for coherent demodulation, includes applying the estimation filter to the received signal in order to obtain the channel estimates. The filter may be equivalently applied in the time or frequency domain or both (jointly). Some channel estimator implementations may not be based on the three steps described above, but still exploit the same principles. For instance, the three steps may be carried out on reference signals (RS) within a single sub frame only.
Accurate estimation of the filter parameters in the first step leads to improved channel estimation. Even though the UE may obtain filter parameters from observation of the channel over a single sub frame and for one RS port, improved filter parameters may result in improved estimation accuracy by combining measurements associated with different antenna ports, i.e., different RS transmissions, sharing similar statistical properties. Furthermore, the channel estimation accuracy may be improved by combining RSs associated with multiple physical resource blocks (PRBs). Note that the network is typically aware of which RS ports are associated with channels with similar properties, based on knowledge of how antenna ports are mapped to physical points.
The UE is also aware a-priori of such information because of the principle of quasi co-location (QCL) as specified in Rel.11. A UE may not assume that two antenna ports are QCL unless specified otherwise. The following antenna ports can be assumed to be QCL with respect to the listed properties.                In TM1-TM9 and in case of TM10 with QCL configured to type A:                    CRS, DMRS and CSI-RS are QCL with respect to Doppler spread, Doppler shift, delay spread and average delay;            All CRS ports are also mutually QCL with respect to average gain; and            All CSI-RS ports are also mutually QCL with respect to average gain;                        In TM10 with QCL configured to type B:                    DMRS and CSI-RS are QCL with respect to Doppler spread, Doppler shift, delay spread and average delay; and            CSI-RS and CRS are QCL with respect to Doppler spread and Doppler shift.In the TM10 Type B configuration (which is configured using RRC signaling), the CRS is QCL with CSI-RS and DMRS only with respect to Doppler shift and Doppler spread. The channel analyzer in the UE can thus estimate these parameters from the CRS and then use the estimated parameters when determining the channel estimation filter. However, the delay spread and average delay must be estimated using the CSI-RS since the CRS is not QCL with the DMRS for these parameters. For TM1-9, the channel analyzer can estimate all the parameters from CRS, interpolated across multiple sub frames, prior to receiving any scheduled data.                        
Each antenna port is represented by a set of RS in the OFDM time frequency grid, as shown FIG. 3. When the channel estimator filter has been determined according to step 1 and step 2 discussed above, the third step of channel estimation should be performed. The channel estimation is improved when more resource elements (REs) are taken into account in the interpolation/extrapolation filtering of the estimates obtained based on the REs containing the corresponding RS. However, there are some limitations on the extent to which the filtering in channel estimation step 3 can be done:                A channel estimator for CRS may use CRS RE from any downlink sub frame and from any RB in each sub frame;        A channel estimator for DMRS may use DMRS RE within one sub frame only and only within a group of RBs known as the physical resource group (PRG). The PRG is 1, 2 or 3 RB depending on the system bandwidth, transmission mode and whether PMI feedback is configured; and        A channel estimator for CSI-RS may use CSI-RS RE within the sub frame only and from any RB in the sub frame.A terminal that does not rely on CRS at all has also been proposed. However, when the carrier is operated without CRS, which provides performance gains, most transmission modes become unavailable (only TM9 or TM10 is possible) since they rely on CRS for demodulation and channel state information feedback. This implies that transmit diversity transmission (TM2) or open loop MIMO (TM3) becomes unavailable. Transmit diversity and open loop MIMO are particularly useful in high speed scenarios and in other scenarios where the closed loop feedback modes have poor performance as when the periodicity of the CSI-RS is long to minimize the downlink transmissions and maximize the DTX lengths. Further, when DTX of downlink sub frames is applied, CRS is no longer available in every sub frame and transmission modes that use CRS may not be available.        
Therefore, current proposals do not provide for use of discontinuous transmission modes where CRS are provided in only some sub frames when a transmission mode is one of transmit diversity mode and open loop multiple-input-multiple-output (MIMO) mode.